Source coupling ratio (SCr) is a common performance measurement for flash cells and particularly so-called split cells or split gate cells. A high source coupling ratio provides for higher erase speeds and a higher programming speed. As the dimensions of split cells decrease, however, the source coupling ratio tends to decrease as well. In conventional split cell design, if one increases the source junction width in order to increase the SCr, the relative channel length is proportionately reduced, resulting in increased punchthrough effects and poor program efficiency.
Various approaches have been attempted to increase the efficiency and performance of flash memory cells. U.S. Pat. No. 6,406,961 B1 to Chen describes a process for producing flash memory without misalignment of the floating gate with field oxide. U.S. Pat. No. 6,124,609 to Hsieh et al. describes a split gate flash memory with a buried source to shrink cell dimensions and increase the coupling ratio. U.S. Pat. No. 6,046,086 to Lin et al. describes a method to improve the capacity of data retention and increase the coupling ratio of the source to the floating gate in split-gate flash. U.S. Pat. No. 6,017,795 to Hsieh et al. also describes a method of fabricating a buried source to shrink cell dimensions and increase the coupling ratio in split-gate flash memories.